Quercus rotundifolia, a charming shrub or small tree with evergreen leaves, is more commonly called the sweet oak, continental oak, ballota oak or holm oak (a moniker it shares with the much larger Quercus ilex of which it was previously thought to be a variety). As a member of the Fagaceae botanical family, Quercus rotundifolia is related to beech and chestnut trees. The genus this shrub falls into, Quercus, represents the oak branch of this larger family. To date, around 500 distinct oak species have been identified. Some of these have deciduous leaves, but the holm oak is an evergreen.
This modestly-sized oak has several things going for it — not only is Quercus rotundifolia, as a much smaller oak, suitable for compact gardens, it can also withstand a surprising range of temperatures, from very cold to extremely hot, and thrives at all altitudes.
While the holm oak is a versatile shrub or tree that does not need a lot of care, people looking to grow Quercus rotundifolia in their gardens should familiarize themselves with the tools they need to help their holm oak stay healthy and strong.
About Holm Oak (Quercus Rotundifolia)
- Quercus rotundifolia is an evergreen oak tree or shrub with dense, deep green, foliage. Most gardeners treasure this tree for its beauty, but holm oak can also be exceptionally useful as a living privacy hedge, or to create a garden that is sheltered from heavy winds.
- Holm oak is native to the Iberian peninsula (Spain and Portugal), and also grows in nearby North African countries like Morocco, as well as in southern France. As Mediterranean trees, holm oaks prefer climates with warm summers and colder winters, but they can also be grown in more northern regions such as the UK.
- It often happens that multiple different (and sometimes completely unrelated) plant species go by the same common name. That is why we always have to refer to the scientific name, too. However, it is no coincidence that Quercus rotundifolia shares the name holm oak with Quercus ilex — this particular species used to be classed as a variant of Quercus ilex, namely as Quercus ilex var. ballota. Although this oak, also called the ballota oak, is now recognized as a unique species, this history may create some confusion if you are trying to get a ballota oak from a nursery.
- Like other oak species, the holm oak has traditionally been used in folk medicine, specifically for its tannins. The leaves, bark, and seeds can all be used to create antifungal and antibacterial remedies.
- The seeds of the holm oak are edible, both in their raw and cooked form, and are often used to make tea.
- Holm oak trees or shrubs sometimes fall victim to oak wilt disease and chestnut blight, both of which are fungal diseases. These diseases do not usually threaten the tree’s life, and other than these, holm oak trees are fairly resistant to pests and diseases. In recent times, fungal root rot has become a problem due to weather extremes.
- Oak trees produce acorns, which pet lovers should know are toxic to dogs. Because dogs are very tempted to play with these interesting fruits, it may not be a good idea to have a holm oak in your garden if you share your home with a dog.
- Holm oak trees have become less abundant in their native Portugal recently, as a result of events like wildfires and the clearing of their native lands to create vineyards, despite the fact that the tree has protected status in the country. By adding a Quercus rotundifolia to your garden, you’d help to keep this species going!
Holm Oak (Quercus Rotundifolia) Features: An Overview
- Quercus rotundifolia is a wonderful evergreen that can grow as a tall shrub or a small tree. The holm oak produces charming catkins, as well as acorns. Holm oaks are usually added to gardens as ornamental trees, and they are slow growers.
- On average, these trees grow to be around 26 to 39 feet (eight to 12 meters) tall, though they can sometimes grow taller but may also be much more compact in size. The holm oak can develop an impressive spread, becoming wider than it is tall, with branches that create a beautiful canopy.
- The gray-brown bark of the holm oak has a cracked and roughly textured surface.
- The evergreen leaves of the holm oak can vary rather a bit in shape and many be oblong, elliptical, or rounded. They generally have a simple shape with spiny edges, and are hairless. The leaves of the holm oak have a glossy sheen and are typically between half an inch and an inch and a half (one and a half to four centimeters) long.
- Quercus rotundifolia produces both male and female flowers, but the male flowers are most aesthetically interesting. They grow as green-yellow catkins that appear in late spring and last through early summer. These catkins hang down from the tree in large clusters, and later fall down.
- The holm oak’s acorns aren’t only beautiful to look at, making their appearance between October and November, but they can also be eaten. The most common culinary use is likely in bread, where the acorns can be incorporated into complex nut breads. They can also be ground into a paste or used to make tea. The unripe fruits have a fresh green, yellow, and brown color, while the acorns turn a deep brown shape as they mature. A single seed can usually be found inside each acorn.
- Finally, oak galls can sometimes be seen on the holm oak. These are semi-spherical, irregularly-shaped, growth that result from insect activity that is otherwise harmless to the plant. These oak galls add curiosity value to gardeners, but have also been used for medicinal reasons in the past.
Growing Holm Oak or Ballota Oak Trees
The holm oak is an especially hardy tree that can thrive in a variety of conditions. This tree prefers to grow in full sun, with access to at least six daily hours of bright light, but you don’t necessarily have to live in a warm Mediterranean climate for the holm oak to perform very well in your garden. These trees can tolerate cool temperatures of 5 °F (-15 °C) or even slightly colder, but also love hot summer temperatures of around 118 °F (48 °C). Gardeners hoping to grow Quercus rotundifolia in areas with much cooler summers can still do so, but the acorns the tree produces are unlikely to ripen fully in this case.
In their native zones, holm oaks can usually be found growing quite densely together, in areas with poor soil that isn’t of value to farmers. Quercus rotundifolia is adapted to a shocking broad range of soil types, however — it can grow well in clay, loam, sandy, or chalky soil. Having said that, if you can pick, holm oak trees do prefer a nutrient rich soil. You will not have to worry about pH levels.
The one soil condition Quercus rotundifolia will absolutely not tolerate is soggy, poorly-drained, soil. The holm oak’s root systems become extremely vulnerable to fungal root rot in this case, a condition that can easily cause them to perish.
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It is also interesting to note that holm oak trees are extremely wind-resistant, and are therefore often chosen as a wind barrier in coastal gardens, especially in the Mediterranean.
It is not necessary to feed your ballota oak fertilizer. To protect it against pests, however, it is a good idea to make a natural mulch out of their dried and brown fallen leaves. Fresh leaves should be removed from the ground, as they steal away some of the nutrients in the soil as they decay.
Gardeners should prune old branches that are in danger of falling to protect themselves and others, and can also prune the tree, during its dormant stage, to impede its growth, if they would prefer to grow holm oak as a large shrub rather than a smaller tree.
In the fall, gather up the acorns to use them to propagate the holm oak, as a decorative feature, or to use in the kitchen! Alternatively, allow squirrels and other small animals to enjoy the fruits Quercus rotundifolia bears, which will offer you the chance to take plenty of lovely pictures.
Watering Holm Oak
Young saplings should be watered frequently to give them the best chance, but should not become waterlogged. Mature holm oaks are very drought resistant and will not usually require any supplemental watering at all. Overwatering is the biggest threat here — to protect your holm oak from fungal diseases, make sure not to offer them too much water.
Propagating Holm Oak (Quercus Rotundifolia)
If you would like to propagate your holm oak, you have a few different options at your disposal! You will have two different opportunuties to take cuttings each year, and can also start Quercus rotundifolia from seed if you are able to follow the right steps.
Let’s take a look at your options, starting with simply planting acorns — the fruit of the holm oak, which usually harbors one seed, and on occasion two — directly into your garden:
- Collect a whole bunch of acorns, picking ones that seem undamaged, after they drop from your ballota oak sometime in the fall. Once you have collected many more than you intend to plant, fill a bucket with water and place the acorns inside. Those that are in good condition will sink to the bottom, while damaged acorns float to the top. Now, select some healthy acorns to plant.
- Like birds such as jays do, “bury” (plant) the acorns just below the surface of the soil.
- As the acorns sprout, keep the soil in your garden moist at all times, but do not drown the acorns.
- When winter arrives, leave the acorns alone.
- Keep watching to see which ones succeed!
If you have squirrels in your area, you can attempt to guard the potential young seedlings from these avid collectors by placing small pieces of chicken wire under the ground. Squirrels also dislike the smell of cayenne pepper, which you can use to your advantage if you like.
If you are planning to start your acorns in pots indoors, pay attention — you’ll need to take a few additional steps:
- Collect acorns and determine which ones are healthy, as described before.
- Once you have done that, place your acorns in the refrigerator for 12 weeks. In nature, the acorns rely on cool fall weather to activate them, but they won’t have access to cool weather indoors. People living in climates where it does not get cold can use this strategy, too.
- Place your acorns in the fridge immediately after collecting them if you are not planting them in the garden right away — the seeds of the holm oak dry out very quickly, after which they will not be able to grow into seedlings.
- When started indoors over the winter, after the 12 week cold period, it will take anywhere from two to six weeks for the seeds of the acorn to start sprouting. Remember to keep their soil moist but well-draining.
You can instead opt to propagate Quercus rotundifolia through cuttings, if you prefer. Semi-hardwood cuttings can be taken in the summer, and hardwood cuttings during the winter. Follow the same steps you would for any other cutting, and dipping the cutting in rooting hormone powder is recommended. Successful cuttings can take between a month to three months to take root.
If you are planning on starting your cutting or seeds indoors, you should keep in mind that holm oak trees do not tolerate being transplanted very well. The young saplings should be placed outdoors in the garden as soon as possible, during the spring, and should not be left in a pot for more than two years.
Quercus rotundifolia is an oak that really doesn’t receive enough attention outside of its native zones — and that means that, if you add holm oak to your garden, you will have an eye-catcher on your hands! These trees do not need a lot of care, and are easy to grow.